Semiconductor device inspection apparatus

ABSTRACT

A semiconductor device inspection apparatus having a noise subtraction function includes an electron gun, a stage for holding a sample, a main detector for detecting a signal discharged from the sample, and at least one or more sub detector for detecting noise generated from the sample or apparatus so that there can be obtained an image in which the noise caused by discharge generated on the sample or in the apparatus is removed from the signal. The noise subtraction function subtracts the noise detected by the sub detector from the signal detected by the main detector to remove or reduce the noise from the signal.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device inspectionapparatus and more particularly to a semiconductor device inspectionapparatus for inspecting a defect existing in patterns formed on asemiconductor wafer.

BACKGROUND OF THE INVENTION

Semiconductor devices such as memories or microcomputers used forcomputers are manufactured by repeating the steps for transferring,using the exposure, lithography and etching processes, a pattern ofcircuits formed on a photomask. During the manufacturing process ofsemiconductor devices, the quality as a result of the lithography,etching and the other processes, and the presence of defects such asgeneration of foreign matters greatly affects the manufacturing yield ofthe semiconductor devices. Accordingly, to detect the presence ofabnormality or defects at an early stage or even before they occur, aninspection of the patterns on the semiconductor wafer is performed upontermination of each manufacturing process. As one example of a methodfor inspecting defects existing in a pattern on the semiconductor wafer,a method and an apparatus for inspecting patterns using an electron-beamimage have been put to practical use.

U.S. Pat. No. 6,583,414 B2 discloses an inspection apparatus including asection for irradiating and scanning an electron beam to a surface of asubstrate where a circuit pattern is formed; a detecting section fordetecting signals secondarily generated from the surface of thesubstrate; a section for forming an image on a display device from thesignals detected by the detecting section; and a section forautomatically recognizing information about foreign matters and defectsbased on figure, roughness and voltage contrast of the surface of thesubstrate from the image. These inspection apparatuses must obtain animage with a higher SN ratio at an extremely high speed in order toperform a high-throughput and high-accuracy inspection. To satisfy sucha requirement, it is required to keep a higher SN ratio by ensuring thenecessary number of electrons to be irradiated to the circuit patternusing a large-current electron beam which is equal to or more than 1000times (100 nA or more) that used for a usual scanning electronmicroscope (SEM). Further, it is essential to highly efficiently detectsecondary electrons produced from a substrate and reflection electronsreflected therefrom at a higher speed.

During the detection of defects, when the originally-unintended defectsare detected by the noise caused by various reasons, it is expected thatthere is caused deterioration not only in the classification accuracy ofthe automatic defect recognition but also in the inspection performanceitself of the inspection device. Therefore, as disclosed in JapanesePatent Application Laid Open No. 2002-124555, there is used a method inwhich defects are detected and then automatically classified to therebydetermine whether the defects include a real defect or falseinformation. Further, as disclosed in U.S. Pat. No. 7,230,723 B2, thereis also used a method in which noise randomly generated is statisticallyindexed and reduction in the noise is performed by the image processing.

SUMMARY OF THE INVENTION

However, it is not enough to perform an automatic defect classificationmethod or statistical image processing disclosed in Japanese PatentApplication Laid Open No. 2002-124555 or U.S. Pat. No. 7,230,723 B2 asdescribed above. For example, when the discharge is generated in aminute region due to charging caused by a large-current electron beamirradiated to a sample, a large number of electrons or electromagneticwaves are generated from the discharge portion to be detected, therebybeing erroneously recognized as defects on a device in some times.Further, in the SEM-type inspection device, since an extremely highelectron beam is controlled using some different types of high voltageelectrodes, noise caused by discharges or noise caused byelectromagnetic waves generated from the high voltage electrodes isdetected using the detector, thereby being erroneously recognized asdefects in some times. Since the noise may always occur at random timeseven on the sample or under the apparatus conditions as described above,it is difficult to determine differences between usual defects and thenoise. Therefore, it is very difficult to automatically classify defectsas erroneous recognition of the defects only from information of images.Accordingly, an object of the present invention is to provide asemiconductor device inspection apparatus capable of obtaining an imagein which the noise caused by the discharge generated on the sample or inthe apparatus is removed from the signal.

In view of the foregoing, it is an object of the present invention toprovide an apparatus for disposing a sub detector for detecting thenoise in addition to a main detector for detecting the signal, and forimproving the detection performance of defects using a function ofsubtracting the noise from the signal. That is, the above-describedobject of the present invention can be achieved by the followingconstitution.

-   (1) A semiconductor device inspection apparatus having a noise    subtraction system, comprising:

an electron gun for discharging an electron beam;

a stage for holding a sample;

a main detector for detecting a signal discharged due to an electronbeam irradiated to the sample; and

at least one sub detector for detecting noise generated from the sampleor from the apparatus including the sample, wherein:

the sub detector is constituted by an antenna; and

the noise subtraction system subtracts noise detected by the subdetector from a signal detected by the main detector to remove or reducethe noise from the signal.

-   (2) The semiconductor device inspection apparatus according to claim    1, wherein

the noise subtraction system has:

an image display unit for displaying a signal image and noise imageformed based on each of the signal detected by the main detector and thenoise detected by the sub detector; and

a subtraction image processor for subjecting the signal image and thenoise image to a subtraction processing to obtain an image removednoises.

-   (3) The semiconductor device inspection apparatus according to claim    1, wherein

the noise subtraction system has:

a signal processor for inputting the signal detected by the maindetector and the noise detected by the sub detector to a subtractionanalog circuit to subtract the noise from the signal; and

a subtraction image display unit for displaying an image removed noisesbased on a signal outputted from the signal processor.

-   (4) The semiconductor device inspection apparatus according to claim    2 or 3, wherein

the noise subtraction system has a function of adjusting a noise amountto be subtracted.

-   (5) The semiconductor device inspection apparatus according to claim    2, wherein:

the noise subtraction system has a display device including the imagedisplay unit; and

the display device has a subtraction adjustment section capable ofadjusting the noise amount to be subtracted on the display device.

-   (6) A semiconductor device inspection apparatus comprising a noise    subtraction system, comprising:

an electron gun for discharging an electron beam;

a stage for holding a sample;

a main detector for detecting a signal discharged due to an electronbeam irradiated to the sample; and

at least one sub detector for detecting noise generated from the sampleor from the apparatus including the sample, wherein:

each of the main detector and the sub detector is constituted by a solidstate detector; and

the noise subtraction system subtracts noise detected by the subdetector from a signal detected by the main detector to remove or reducethe noise from the signal.

-   (7) The semiconductor device inspection apparatus according to claim    6, wherein

the noise subtraction system has:

an image display unit for displaying a signal image and noise imageformed based on each of the signal detected by the main detector and thenoise detected by the sub detector; and

a subtraction image processor for subjecting the signal image and thenoise image to a subtraction processing to obtain an image removednoises.

-   (8) The semiconductor device inspection apparatus according to claim    7, wherein

the noise subtraction system has a function of adjusting the noiseamount to be subtracted.

-   (9) The semiconductor device inspection apparatus according to claim    7, wherein

the noise subtraction system has a display device including the imagedisplay unit; and

the display device has a subtraction adjustment section capable ofadjusting the noise amount to be subtracted on the display device.

According to the present invention, there can be obtained an image inwhich the noise caused by discharge generated on the sample or in theapparatus is removed from the signal.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus of the present inventioncapable of reducing noises;

FIG. 2 is a schematic diagram of an image formed by signals, an imageformed by noises, and an image obtained by subtracting the noises fromthe signals of the present invention;

FIG. 3 is a diagram of a detection control circuit system of the presentinvention having a subtraction function using an analog circuit;

FIG. 4 is a diagram of a detection system of the present invention forexplaining detection of the signals and noises by bringing a solid statedetector for detecting the signals into close contact with that fordetecting the noises.

FIG. 5 is a schematic diagram of a display device of the presentinvention, in which the subtraction amount of the noises is adjusted andthe adjustment result is displayed.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments according to the present invention will bedescribed in detail below with reference to the figures.

First Embodiment

Noise reduction using an antenna for detecting an electromagnetic wavewill be described below. As shown in FIG. 1, in addition to a maindetector 4 for detecting a main signal 11 discharged from a sample 3, anantenna 5 for detecting electromagnetic noise 10 is disposed. The mainsignal 11 discharged from the sample 3 is detected by the main detector4, while the electromagnetic wave 10 caused by local discharge in thesample 3 is detected as noise by the antenna 5. Next, the detectedsignal is amplified using a signal amplifier 6 and the detected noise isamplified using a noise amplifier 7, respectively. Next, the amplifiedsignal and noise are analog-to-digital converted by analog/digital(hereinafter, denoted as A/D) converters 12 and 13.

FIG. 2 are diagrams obtained by displaying an image on an image displayunit 9 from each of the digital signals. Here, there are shown imagesobtained by simulating a case where a device on which thin lines areformed at regular intervals is observed. An image 20 is an image formedby the signals, and an image 21 is an image formed by the noises. In theimage 20, there is shown an image obtained by simulating a case wherenoise portions 22 and 24 caused by discharge appear as white luminescentspots on the image along with a true defect 26 on the device. On theother hand, the noise portions 22 and 24 caused by discharge mainlyappear on the image 21 formed by the noises detected by the antenna 5.

Subsequently, by a subtraction image processor 8, the image 21 issubtracted from the image 20. As a result, as shown in an image 27,there can be shown an image on which no noise portions 22 and 24 causedby discharge appear. Further, when this image is compared with an imagehaving no defective spot, the inspection on foreign matters or defectscan be performed.

Further, as shown in FIG. 3, during a period until the image is formed,a constitution in which a detector circuit having a function ofsubtracting the noise from the signal is disposed is also enabled.Hereinafter, a subtraction method using the detector circuit having asubtraction function will be described.

First, the main signal 11 discharged from the sample 3 is detected bythe main detector 4, while the electromagnetic wave 10 caused by localdischarge in the sample 3 is detected as the noise by the antenna 5.Next, the detected signal is amplified using the signal amplifier 6 andthe detected noise is amplified using the noise amplifier 7,respectively. Thus far, a constitution in FIG. 3 is the same as that inFIG. 1. Then, the noise after the amplification by the noise amplifier 7is analogously subtracted by a subtraction analog circuit 14 from thesignal after the amplification by the signal amplifier 6. The subtractedsignal is analog-to-digital converted by the A/D converter 15, and thenthe image is displayed, whereby only an image removed noises can bedisplayed on the image. In this case, the subtraction image processor 8is not required. When the subtraction amount is adjusted, thesubtraction amount via a digital/analog (hereinafter, denoted as D/A)converter 17 is analogously adjusted to thereby realize the adjustment.

Second Embodiment

To detect a signal or noise formed by electrons, a solid state detectoror photo-multiplier can be used as a detecting section. Here, a methodof detecting the signal or noise using the solid state detector will bedescribed. In addition, since the solid state detector can detect notonly an electron but also an X-ray and excitation light, the followingmethod and apparatus constitution are the same as in a case of the noiseformed by electrons also when the noise is formed by X-rays andexcitation light.

As shown in FIG. 4, at least two or more solid state detectors aredisposed to thereby realize the apparatus constitution. Control of theelectron beam orbit is performed using an electron lens 61 such that themain signal 11 discharged from the sample enters into the solid statedetector 62 for detecting the signal. Thereby, the main signal 11 can bedetected. In FIG. 4, the electron lens 61 is disposed apart from thedetector 63; however, by applying a high voltage to the detector 63itself, electrons can be also converged while attracting electronsdischarged from the sample 3 to the detector 63. The main signal 11 isdetected by the solid state detector 62 for detecting the signal. On theother hand, since electrons with high-energy discharged from the sample3 or electrons caused by the discharge of high voltage electrodes moveon an electron beam orbit not foreseen in designing of the orbit, thedetection is performed by both the detectors. Specifically, only thenoise is detected using the solid state detector 60 for detecting thenoise. Relating to a method and an apparatus constitution forsubtracting the noise from the detected signal, figures of the presentembodiment are the same as FIGS. 1 to 3.

Further, when solid state detectors can be used as the detector 4 fordetecting the signal and the electron beam detector 60 for detecting thenoise, the detectors can be brought into close contact with each otheras shown in FIG. 4. In this case, it becomes difficult to design theelectron beam orbit for entering the signal discharged from the sample 3into the main detector 4; however, since a slight deviation of the orbitcan be detected, the noise can be effectively obtained.

Third Embodiment

Hereinafter, a method and a function for adjusting the subtractionamount will be described. Depending on the apparatus, sample orinspection conditions, it is different as to how much noise isgenerated. For example, when the inspection is performed by irradiatinga large-current electron beam to an easily charged wafer, chances of thefine discharge become high.

When the subtraction amount is too large or too small, it is consideredthat the noise is still erroneously recognized as a defect. Therefore,the function capable of adjusting the subtraction amount is required. Tosolve the above problem, when the noise is subtracted from the signal, asubtraction adjustment section 52 capable of adjusting the subtractionamount on a display device must be provided as shown in FIG. 5 so thatthe subtraction amount can be adjusted. The optimal subtraction amountcan be found using the subtraction amount adjustment function.

The optimal adjustment amount can be found by the following procedure.First, the subtraction adjustment amount is changed several times toperform an experimental inspection to thereby obtain the number ofdetection defects, respectively. Next, there is prepared a graph 51 ornumerical value table indicating a relationship between the subtractionadjustment amount changed several times and the number of detectiondefects of that time. It can be determined conditions in the subtractionadjustment amount of the minimal number of detection defects in thegraph or table. As a result, the optimal subtraction adjustment amountcan be found out. In the subtraction adjustment method, since the simplesubtraction amount is changed to be simply tested, full automation ofthis adjustment can be also realized.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A semiconductor device inspection apparatus having a noisesubtraction system, comprising: an electron gun for discharging anelectron beam; a stage for holding a sample; a main detector fordetecting a signal discharged due to an electron beam irradiated to thesample; and at least one sub detector for detecting noise generated fromthe sample or from the apparatus including the sample, wherein: the subdetector is constituted by an antenna; and the noise subtraction systemsubtracts noise detected by the sub detector from a signal detected bythe main detector to remove or reduce the noise from the signal.
 2. Thesemiconductor device inspection apparatus according to claim 1, whereinthe noise subtraction system has: an image display unit for displaying asignal image and noise image formed based on each of the signal detectedby the main detector and the noise detected by the sub detector; and asubtraction image processor for subjecting the signal image and thenoise image to a subtraction processing to obtain an image removednoises.
 3. The semiconductor device inspection apparatus according toclaim 1, wherein the noise subtraction system has: a signal processorfor inputting the signal detected by the main detector and the noisedetected by the sub detector to a subtraction analog circuit to subtractthe noise from the signal; and a subtraction image display unit fordisplaying an image removed noises based on a signal outputted from thesignal processor.
 4. The semiconductor device inspection apparatusaccording to claim 2, wherein the noise subtraction system has afunction of adjusting a noise amount to be subtracted.
 5. Thesemiconductor device inspection apparatus according to claim 2, wherein:the noise subtraction system has a display device including the imagedisplay unit; and the display device has a subtraction adjustmentsection capable of adjusting the noise amount to be subtracted on thedisplay device.